An annual survey that asks Americans about crimes they've experienced showed that the rate at which those surveyed said they had been raped or sexually assaulted nearly doubled from 2017 to 2018.

The 2018 National Crime Victimization Survey (NCVS), released Tuesday, is managed by the Bureau of Justice Statistics at the U.S. Justice Department, and asks people if they've been victims of crimes — even if they didn't report them to police.

"We go to about 150,000 households each year, interviewing about 240,00 people," says Rachel Morgan, a statistician at BJS. "We have found consistently that about half or less than half of crimes are actually reported to police, so the FBI are getting different numbers than we are."

The stand-out result in the 2018 survey is the rate at which respondents said they'd been victims of rape or sexual assault. That number nearly doubled over the previous year, from 1.4 per 1000 in 2017 to 2.7 in 2018.

Independent analysts say that jump is the most statistically significant figure in the 2018 report.

Callie Rennison, a professor at the University of Colorado Denver School of Public Affairs and an expert in sex crime victimology, used to work on the government's victimization survey and says it uses a conservative method when counting rape numbers.

"I think the fact that even with that conservative estimate, with the fact that most people aren't willing to share these incidents that we're seeing a significant increase from one year to another is something really worth paying attention to," Rennison said. "It says something important."

But it's hard to pin down what the number really means. Is this a jump in the total number of sexual assaults, or is it a matter of more reporting, a reflection of victims being more willing to talk about those crimes in 2018, at the height of the #metoo phenomenon?

"I think it's both," Rennison said. "I think that people are more willing to share this with interviewers and tell about victimizations they've experienced, but I also think that a part of it could be, is that there has been, among some people, I don't know, this idea that grabbing people — which is sexual assault — or raping, isn't a big deal. And so we might be seeing just actual more offenses, as well."

The survey shows victimization rates for other violent crimes are up too. The increases are much smaller, but the fact that they've increased three years in a row has some people talking "trend."

That impression is reinforced by the opening line of the BJS's report on the survey, "The longstanding general trend of declining violent crime in the United States, which began in the 1990s, has reversed direction in recent years."

"Has it gone up for the past two years, three years? Yes. Is that something to keep an eye on? Absolutely. Is it still at almost record lows? Yeah, it's lower than it was in 2012, lower almost about where it was in 2013," said John Pfaff, a law professor at Fordham University.

Pfaff says when crime rates are historically low, year-to-year increases shouldn't be that surprising — and just because you start to see an upward trend, it shouldn't become an argument against the current movement for criminal justice reform.

Jeff Asher, a crime analyst and founder of AH Datalytics, says the information the victimization survey compiles is important, but "you can't really take NCVS by itself as the thing that tells you the story of crime in America."

According to Asher, the victimization survey's numbers have to be combined with the FBI's Crime in the U.S. Report to get an accurate picture of U.S. crime rates.

"Somebody that says that they've been the victim of a crime in the survey may not meet the standards of what the actual crime is when the law enforcement is reporting it," he said. "It's helpful to have the police give the numbers, but it's also helpful to have NCVS, which provides a fuller generalized picture of crime in America."

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STEVE INSKEEP, HOST:

Some other news now - violent crime rates are up for the third year in a row. NPR's Martin Kaste reports on the annual National Crime Victimization Survey.

MARTIN KASTE, BYLINE: Every year, police departments send their crime stats to the FBI, which releases a national report. This isn't that. This is an annual survey which asks Americans about the crimes they've experienced. And as surveys go, it's a big one.

RACHEL MORGAN: We go to about 150,000 households each year, interviewing about 240,000 persons.

KASTE: Rachel Morgan is with the Bureau of Justice Statistics. She says they ask people directly if they've been victims of crimes, even crimes they've never called the cops about.

MORGAN: We have found consistently that about half or less than half of crimes are actually reported to police. So the FBI are getting different numbers than we are.

KASTE: And in the survey for 2018, one number in particular stands out - the rate at which people said that they'd been victims of rape or sexual assault. That number nearly doubled over the previous year. Callie Rennison is a professor at the University of Colorado Denver's School of Public Affairs, and she's an expert in sex crime victimology. She also used to work on the government's Victimization Survey, which she says actually uses a conservative method when collecting rape numbers.

CALLIE RENNISON: I think the fact that even with that conservative estimate, with the fact that most people aren't willing to share these incidents, that we're seeing a significant increase from one year to another is something really worth paying attention to. It says something important.

KASTE: But what this number is saying exactly is hard to pin down. Is this a jump in the total number of sexual assaults or is it a matter of more reporting, a reflection of victims being more willing to talk about these crimes in 2018, at the height of the #MeToo phenomenon?

RENNISON: I think it's both. I think that people are more willing to share this with interviewers - right? - and tell about victimizations they've experienced. But I also think that a part of it could be is that there has been, among some people - I don't know - this idea that grabbing people, which is sexual assault, or raping isn't a big deal. And so we might be seeing just actual more offenses as well.

KASTE: As to the rest of the survey, it shows that victimization rates for other kinds of violent crime are also up slightly. It's the third year that they've gone up, though, which has some conservatives calling for more policing and advocates for criminal justice reform pushing back. John Pfaff is a law professor at Fordham University, and he says you have to put this violent crime rate increase into context.

JOHN PFAFF: Has it gone up for the past two years, three years? Yes. Is that something to keep an eye on? Absolutely. Is it still at almost record lows? Yeah, it's lower than it was in 2012, lower almost about where it was in 2013.

KASTE: Pfaff says when crime rates are historically low, year-to-year increases shouldn't be that surprising. And just because you start to see an upward trend, that should not become an argument against the current movement for criminal justice reform.

Martin Kaste, NPR News.

(SOUNDBITE OF HADOKEN'S "TIME AND THE OBSERVER") Transcript provided by NPR, Copyright NPR.